Leaders of the 16 nations of the Commonwealth have changed legislation governing the line of succession to rescind a law that favours boys over girls in the royal line.
Prime Minister David Cameron made the announcement alongside Australian Prime Minister Julia Gillard at Perth, Western Australia, on Friday.
The 16 “realms” also agreed to remove the ban on a spouse of a Roman Catholic taking the throne.
Since the 1701 Settlement Act, sons of the monarch have taken precedence over daughters.
David Cameron is expected to introduce legislation in the UK to ensure equality within the succession for any offspring of the Duke and Duchess of Cambridge.
Speaking ahead of the Commonwealth summit Australia, Cameron said the laws were "outdated".
Following the amendment, Royal succession will be determined by order of birth.
Other laws will also be amended, including the barring of anyone that marries a Roman Catholic from succeeding the crown.
Catholics are still banned from the throne, but a future monarch can now marry a Catholic and remain head of the Church of England.
Future royalty will no longer need the monarch's consent to marry.
In his address, Cameron told Commonwealth leaders: "The idea that a younger son should become monarch instead of an elder daughter, simply because he is a man, just is not acceptable any more.
"Nor does it make any sense that a potential monarch can marry someone of any faith other than Catholic.
"The thinking behind these rules is wrong. That's why people have been talking about changing them for some time. We need to get on and do it."
However, campaign group Republic has criticised the amendments to the Act of Succession, branding the changes "crass and absurd".
Republic's Campaign Manager, Graham Smith, said:
"These proposals change nothing of substance, even if William and Kate have a daughter. All this fuss is about a trivial detail of a succession that may or may not happen in 70 years time."
"The monarchy discriminates against every man, woman and child who isn't born into the Windsor family. To suggest that this has anything to do with equality is utterly absurd. It fails the equality test both in practice and in principle."
"In practice it simply means that the eldest child of one family is preferred over all others. Inequality is therefore further entrenched in the system."
"In principle all children in Britain should have an equal opportunity to stand for the position of Head of State. Anything short of that is an affront to the principles of equality."
Smith added that the monarchy is founded on discrimination and elitism. "It has no place in a debate over equality of opportunity," he said.
At the press conference, Cameron also mentioned the ongoing Occupy protest outside St Paul's Cathedral, saying he was “very concerned” that the building was still not open to the public because of the continuing protest. He also called for the matter to be resolved.
The Queen is also in Perth ahead of the summit.
The changes were made hours after the Duchess of Cambridge attended her first solo engagement as a Royal at a private diner for the charity Kind Direct. Prince Charles was due to attend but had to pull out due to a rearranged flight to Saudi Arabia.
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